1. Field of the Invention
The present invention relates to a numerically controlled grinding machine for grinding a tapered surface of a workpiece.
2. Description of the Prior Art
In a numerically controlled grinding machine of this kind, a wheel head and a table, which are movable along X axis and Z axis perpendicularly intersecting each other, are provided on the bed, and a workpiece is rotatably supported on the table in such a way that the rotational axis of the workpiece inclines with respect to the Z-axis by a predetermined angle which is equal to an angle formed between the tapered surface and the rotational axis of the workpiece. The tapered surface is ground by a grinding wheel supported on the wheel head.
In such grinding machine, the workpiece is supported on the table, for example, with a tapered mandrel having a tapered portion, whereat the inner tapered hole of the workpiece is engaged. In that case, the axial position of the workpiece changes due to the deviation of the diameter of the tapered hole of the workpiece. As a result, the distance in the X-axis direction between the grinding surface of the grinding wheel and the tapered surface of the workpiece also changes. Consequently, the machining error in the diameter of the ground tapered surface becomes large, unless the feed amount of the wheel head along X axis is compensated for according to the deviation. Prior art grinding machines, however, do not equip any apparatus for compensating such deviation. Even if the workpiece is supported on the table without using any mandrel, the same problem would occur when the axial position of the workpiece changes due to other factors.
To overcome this disadvantage, a grinding machine described in the U.S. Pat. No. 4,584,795 has been proposed. In the grinding machine described therein, a measuring device having a feeler, engageable with a reference end surface of a workpiece, is mounted on the bed in order to detect the positional deviation of the workpiece. The numerical control apparatus compensates a feed amount of the wheel head, which is used in the grinding operation for the tapered surface, depending upon the detected positional deviation, whereby the tapered surface can be ground to a predetermined diameter regardless of the positional deviation of the workpiece. The grinding machine however, has the disadvantages described below. First, the swivel table supported on the traverse table should be pivoted back to the original position, whereat the rotational axis of the workpiece is parallel to the Z-axis, for detecting the positional deviation of the workpiece, and then the swivel table should be pivoted to a predetermined angular position in order to grind the tapered surface of the workpiece. Accordingly, the machining time becomes longer than that of the conventional grinding machine because of the pivoting operation of the swivel table.